I do experience a really annoying problem with TestNG and RESTeasy.
I do have a class that runs several tests against an API class which uses the RESTeasy framework
Unfortunately I can't tell you why this happened, but I can tell you how to get around this issue.
The problem was, that PowerMockito scanned tha class path and also added the RESTeasy classes (which are located within the package 'javax.ws.*'. Therefor the above mentioned RuntimeDelegate was loaded by the PowerMockito classloader and caused later the issue, that the class was compared against one from a different classloader.
To get around this issue, tell PowerMockito to ignore the javax.ws package when scanning for classes:
@PowerMockIgnore({"javax.ws.*"})